Laminated stretched film



April 22, J. E 5NYDER ET AL LAMINATED STRETCHED FILM Filed July 21, 1950UNSTRETCHED FILM LAITERALLY LONGITUDINAL STRETCH 0 A I FILM E L M NAT'ONSTRETCHING UNSTRETCHED FILM FIG. I

UNSTRETGHED FILM LONGITUDINALLY LATERAL STRETCH"D LAMI A IO FILM N T NSTRETCHING UNSTRETCHED FILM FIG. 2

INVENTORS JAMES E. SNYDER BY FOSTER J. YOUNG ATTORNEY Patented Apr. 22,1952 2,594,229 LAMINATED STRETCHED FILM James E. Snyder, Akron, andFoster J. Young, Cuyahoga Falls, Ohio, assignors to WingfootCorporation, Akron, Ohio, a corporation of Delaware Application July 21,1950, Serial No. 175,254 4 Claims. (Cl. 154-124) This invention relatesto a. laminated film of rubber hydrochloride which has high strength atlow temperatures while retaining low permeability to moisture vapor. Itis relatively stiff so that it can be fed easily into a packagingmachine. It has little or no plasticizer content. It is formed from twoouter plies of unstretched film with an intermediate ply of filmstretched in one direction. These plies are laminated by heat andpressure and the whole is stretched perpendicularly to the aforesaiddirection. The outer surfaces of the resulting film have been stretchedin one direction, the intermediate portion has been stretched in bothdirections.

With theadvent of frozen foods there has been a demandfor a film whichis not brittle but has,

high strength at low temperatures, so that if a package offfrozen foodswrapped in the material? is dropped or hit, the film will not be brokenbutwill remain intact. Unplasticized rubber hydro: chloride film, madein the usual manner by casteing a cement or solution of rubberhydrochloride on a film-forming surface and then evaporating. thesolvent becomes brittle at low temperatures. This unplasticized film istherefore not at all satisfactory for the wrapping of frozen foods. Itis well known that the addition of plasticizer to a rubber hydrochloridefilm strengthens the film and somewhat different properties are obtainedby adding different plasticizers. Certain plas-l ticizers improve thetrength of the film at low temperatures but the addition of anyplasticizer decreasesthe moisture-vapor proofness of the film. The filmsof this invention are free from plastiizer ,or' contain only a smallamount of plasticizer so that the resistance of the film to the passageof water vapor at least approaches that of unplasticized rubberhydrochloride film.

To test the suitability of a film for use at low temperatures it is heldtaut between 6 inch diameter clamps. A 1 inch diameter steel ball isthen dropped on the sample from different ,heights. 'The height abovewhich the sample ruptures, and below which it resists the impact,

will be referred to herein as drop-height and the resistance offilm tobursting under such condition will be described as the films'drop-heightresistance.

Ordinarily; unstretched cast film (formed by evaporating solvent from acast solution of rubber hydrochloride) has relatively low drop-heightresistance. 7 It has been observed that the dropheight resistance of afilm is increased by stretching it. For instance, a film which has beenstretched to both three times its length and three times its width hasappreciably greater dropheight resistance than unstretched film,particularly at low temperatures and has been used satisfactorily forpackaging frozen foods. However, such film is quite costly. Thelaminated film of this invention has good drop-height resistance at lowtemperature and is cheaper to manufacture than film which has beenstretched in both directions.

References to stretching herein, means sufilcient stretching to producesubstantial orientation of the molecules or molecular aggregates in thefilm, i. e., stretching at least one hundred per cent, and usuallyseveral hundred per cent, up to six or eight hundred per cent or more.

The invention will be further described in connection with theaccompanying flow-sheets which illustrate two general methods ofproducing the film of this invention. Fig. 1 indicates how the film maybe produced by laminating three plies of film and stretching thelaminated, sheet longitudinally, the two outer plies being unstretchedand the middle ply beingstretched laterally before the lamination andlongitudinal-stretching.

Fig. 2 illustrates the converse operation in which the laminated sheetformed from. two outer plies of unstretched film and'an intermediate plyof longitudinally stretched film, are stretched laterally afterlaminating.

Various machines have been devised or suggested for stretching filmeither longitudinally or laterally. It i desirable to heat the film tosomewhat below its melting point before stretching in either direction,and because rubber hydrochloride film is thermoelastic it must be cooledin the stretched condition. The simplest means for stretchinglongitudinally is to pass the filth over a heated roll and from herecarry the heated film through two rolls (which may be cool or cooled)having a higher surface speed. The heated film is stretchedlongitudinally by an amount relative to the difference in the speed ofthe rolls. Equipment .for lateral stretching is described in Young2,473,404. Any suitable equipment may be used for the stretching opera'-tions referred to herein.

In the preferred equipment for lateral stretching a series of arc-shapedrollers is used. .138.011 of these rollers is covered by a rubbersleeve. vas

the roller are turned thefiistance between their ends outside of the arcis always greatertha'n the distance inside of the are. Thesearsed-rollers are placed in a horizontal plane adjacent one another. Thefilm is heated and thenpassed between the first two rollers. It is keptin contact with the surface of the second roller, coming first intocontact with the shorter diameter of the second roller. The roller isrotated in the direction that the film is moving, and because the filmis in contact with the rubber sleeve it is stretched laterally as thesleeve stretches laterally in passing from the shorter side to thelonger side. of'gtheroller. As thefiim reaches the long sideo'f' thisroller, it is transferred-to "the short side of the third roller. It isagain stretched laterally'as it moves from the short to the long side ofthis roller. It is then transferredto-the fourth roller. Thus, bycontinuous stretching in contact with the rubber-sleeve of"eachsuecesiveroller, as the sleeve is stretched inbe'ing rotated from the shortto'the'long side of its roller, the film is stretched to any desiredextent in the crosswise direction.

Stretching tends to orient the molecules or molecular aggregates. Thisproduces a. grain which runs in the direction in which the film hasbeen-"stretched. The more easily'torn along the-grairi than ac'rossthe'gr'ain. After the filmhasbee'n' stretched several hundred percentit'ca'n be torn much more easil y along' the. grain than theun'stretchedfilm. Conversely; itis more 'dlfii'cult to tear" it across"thef'grain.

"In producing a preferred film of this invention three'films are heatedand laminated, and then stretched. Before lamination the. outer plies fare unstretched, and the intermediate 'ply is stretched longitudinally."The films may be heated "n any usual manner, as by being passedoverhct'd rums. and the. fiimsare preferably .lamina'ted by pressurebetweenlheate'd pressure rolls beforefthey enter tneraterm stretchingdevice. In this lateral' stretching device the molecules .or molecularaggregates :of which the outside unstretehe'dplies are composed, areoriented late erally,' andth'e molecules or aggregates of theintermediate ply which. are: oriented longitudinally before it entersthe lateral "stretching device are at least partiall reoriented.

"se'x'reral laminated-films were made in this. en erarmanner. 1 For someu'nexplain'ed reason, each resisted decomposition light 'for a greaterperiod than unstretc'hed film of the 's'ame composi-'' tion.i'Thedrop-hei'ghts given for these test films are to be oom-pared with adrop-height resistance of about 1 inch at 20 Fiforunstretohed, un-

plfistitizod film about .001 inchthick- These severa1-:-rutberhydrochloride films will be de scribed more particularly in whatifollows.

Exempted .The outside pliesvof ithisafilm were each .0012

'in'chthi'ck-zand they econtained no plasticizer. The intermediate ply.likewisenontained no plas ticizer. -It was but .0008 inch thick :andwas formed by; plying up three plies of film each .0008 men thick andthese through. a longitudinalistretcherain whichthey were laminated toone another by heat-andpressure,andstretohed "to about threeitimes"their :original length.

stretched to about: three-.and one half times their'original "width andreduced in gauge toan overall thickness .of .0009'inch. This filmhad' adrop-heightl resistance of .24 inches at- 20 names water-'Vapor-transierrate of -l.---gram's .per-= 1'00 sq. inches in 24 hours, measured by astii ndardteSt. Ils -had a life-pf *92' hours in the Fadeometer, ascompared to a usual life of about 40 hours for ordinary cast film ofthis thickness.

Example 2 This film was prepared from (1) two plies of unstretched film.0008 inch thick containing 2.5 parts of butyl stearate per parts ofrubber hy dro'c'hloride; and (2) an intermediate film about .0015inchthick containing. fi parts of butyl stearate per 100 parts of rubberhydrochloride, prestretched longitudinally about 300 per cent. The

longitudinal ply was prepared by laminating by heat'and stretching threeplies of .0014 inch thick material. These three plies of stretched and--unstretched 'film, laminated together, were then stretched laterallyuntil the lamination was about .0009 inch thick.

film hada drop-height resistance of 48 inches at -20 F. Its water-vaportransfer rate was about 1.3 grams per 100 sq. inches per 24 hours. Itlasted 81 hours in the Fadeometer.

Example-3 I In this .film the outside plies-were each .0008

inch thick and contained no plasticizer. The in-' termediateply was.0014 inch thick and con- Example 4 In this "example the outside plieswere each- .0008'inoh thick and contained 2151 parts of butyl stearateper 100 parts of rubber hydrochloride. The inner ply was .0014 inchthick and'madeof' film containing 2.5 parts of butylstearateper 100:parts of rubber hydrochloride. Three plies unstretched. film of thiscomposition,".0014 inch thick, united by heat, werestretched-longitudinally to three times their original length, to formthis intermediate ply. The unstretched and longitudinally stretchedfilms were laminated by heat and stretched laterally until thelamination was .0009-inch thick.

This film had a drop-height resistance at 20" F. of 46 inches and awater -vapor transfer rate of 1.1 gramsper 100 sq. inchesin 24 hours.

It has been observed that'if bags aremade from the laminated film ofthis invention with the. stretched outer plies lying across the mouth ofthe bag, when heat is applied to" the mouth. after filling, in ordertoseal the mouth, the films shrink somewhat, lessening the length of themouth.- If the bagsbe formed from two rectangularsheets of the laminatedstock or-by'folding a single sheet of the laminated stock, an ear isformedat each end-of thestraight-line seal used to close the mouth. Itis customary, in heat-seal ing stretched film, to maintain-itin thestretched condition until the seal has cooled because'the heating of thefilmcauses it toshrink. The objectionable "ears are eliminated bynot-maintaining theheated mouth of-the bag of-thisqfilm in the stretchedconditionuntil it ;cools..-

It is preferred to use-film in which thereare no more than 10 partsofplasticizer ,per 100-.parts of rubber hydrochloride. Larger amounts ofplasticizer tend to make the film become hazy on stretching, and theyreduce the resistance of the film to the passage of moisture vapor.Butyl stearate was used in the above films because it is particularlyrecommended for use in films to be brought in contact with a foodstuff.Other plasticizers may be used, such as any mentioned in the art for usein rubber hydrochloride. Although in the formulae given the two outsidefilms were in each case of the same composition and gauge, this is notessential but it is believed that such film will have less tendency tocurl than film made from a lamination in which the outer plies are ofdifferent composition or thickness.

In addition to having high strength at low temperatures, the films ofthis invention have high resistance to the transfer of moisture and arefree, or relatively free, from plasticizers and stifienough to be easilyhandled on a wrapping machine. In sunlight they last about twice as longas unstretched film. Each of the three films used in making thelaminated film of this invention preferably range from about .00075 to.001? inch thick and the resultant film will ordinarily be stretched toa thickness of at least about .0007 and may be as thick as .0015 or .002inch up to .0025 inch or thicker. The thicker films are used to obtaingreater resistance to moisture vapor, greater strength or increasedstiffness.

Increasing the thickness of the intermediate ply, which in the endproduct is stretched in two directions, increases low-temperaturestrength proportionally more than increasing the thickness of the outerplies. To improve resistance to moisture vapor permeation, increasingthe thickness and/or reducing plasticizer content of any ply isefiective. The intermediate ply is more expensive than the outer pliesand will be made of the thinnest gauge compatible with the desiredresults. The edges of the laminated sheet are not true, and severalinches must be trimmed from each edge to make a marketable product. Theamount of the more expensive intermediate ply required is reduced to aminimum by using a ply which is narrower than the width of the outsideplies, so that when trimmed, only a minimum of the inner ply is cutaway.

What we claim is:

1. A laminated sheet of rubber hydrochloride film which contains on theaverage no more than 10 parts of plasticizer for each parts of rubberhydrochloride, the sheet being at least .0067 inch thick, the film onboth surfaces being stretched the same amount and in the same onedirection, said sheet between its stretched surfaces consisting of anintermediate layer of rubber hydrochloride stretched at least 100 percent both longitudinally and laterally.

2. The method of producing film which cornprises laminating sheets ofsubstantially unstretched rubber hydrochloride to an intermediate ply ofrubber hydrochloride stretched at least 100 per cent in one direction,and then stretching the lamination at least 100 per cent perpendicularlythereto.

3. The method 01' producing a laminated sheet of rubber hydrochloridewhich comprises laminating rubber hydrochloride film stretchedlongitudinally at least 100 per cent, between two plies of substantiallyunstretched rubber hydrochloride fiim, and then stretching the laminatedsheet laterally at least 100 per cent.

4. The method of producing a laminated sheet of rubber hydrochloridewhich comprises laminating between two plies of substantiallyunstretched rubber hydrochloride film, a ply of rubber hydrochloridefilm which has been stretched laterally at least 100 per cent, and thenstretching the laminated sheet longitudinally at least 100 per cent.

JAMES E. SNYDER. FOSTER J. YOUNG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,004,110 Head June 11, 19352,259,347 Mallory Oct. 14, 1941 2,259,362 Young Oct. 14, 1941 2,335,190Minich Nov. 23, 1943 2,351,350 Mallory June 13, 1944 2,429,177 YoungOct, 14, 1947

1. A LAMINATED SHEET OF RUBBER HYDROCHLORIDE FILM WHICH CONTAINS ON THEAVERAGE NO MORE THAN 10 PARTS OF PLASTICIZER FOR EACH 100 PARTS OFRUBBER HYDROCHLORIDE, THE SHEET BEING AT LEAST .0007 INCH THICK, THEFILM ON BOTH SURFACES BEING STRETCHED THE SAME AMOUNT AND IN THE SAMEONE DIRECTION, SAID SHEET BETWEEN ITS STRETCHED SURFACES CONSISTING OFAN INTERMEDIATE LAYER OF RUBBER HYDROCHLORIDE STRETCHED AT LEAST 100 PERCENT BOTH LONGITUDINALLY AND LATERALLY.
 2. THE METHOD OF PRODUCING FILMWHICH COMPRISES LAMINATING SHEETS OF SUBSTANTIALLY UNSTRETCHED RUBBERHYDROCHLORIDE TO AN INTERMEDIATE PLY OF RUBBER HYDROCHLORIDE STRERCHEDAT LEAST 100 PER CENT IN ONE DIRECTION AND THEN STRETCHING THELAMINATION AT LEAST 100 PER CENT PERPENDICULARLY THERETO.